This invention relates broadly to remote-display circuits for sensing the operating temperature of equipment situated in a region at relatively high electrical potential and for generating, in a region at lower potential, a voltage signal indicative of that temperature. More particularly, it relates to a highly compact and accurate circuit for directly sensing the operating temperature of a high-voltage device and for generating at a remote location an output signal whose frequency in pulses per second equals the measured temperature as expressed in degrees on a selected temperature scale.
This invention was made in the course of, or under, a contract with the Energy Research and Development Administration.
U.S. Pat. No. 3,488,586 describes a circuit for measuring a variable at a selected location and displaying, at a more remote location, an output indicative of the measured variable. The system is designed to generate a d.c. electrical control signal whose magnitude varies with the measured variable. This signal is connected to a pulsed electrical signal whose frequency varies with the magnitude of the control signal. The pulsed signal is converted to light pulses of like frequency, which are transmitted through a light pipe to a remotely disposed receiver. The receiver re-converts the light pulses to an electrical signal of like frequency, and this signal is converted to an output signal indicative of the measured variable. More specifically, the receiver includes a multivibrator which generates a pulse for each of the received light pulses; the pulses from the multivibrator are averaged by a meter to provide a visual indication of the value of the control signal. Alternatively, the multivibrator output is converted to a replica of the original analog control signal.
The present temperature-measuring circuit is an improvement over the above-identified prior circuit in several important respects. First, the sensitivity of the improved circuit is higher by a factor of about 100, yet the circuit is highly stable. Second, the major portion of the input circuit to the above-mentioned light pipe is designed so that it can take the form of a highly compact integrated circuit, one not requiring choke coils or complex SCR-resetting circuits. Third, excellent temperature compensation is provided despite the fact that the signal level is low. In addition, the present circuit permits direct coupling of the thermocouple, whereas the prior circuit intermixes signal and bias to the extent that the thermocouple cannot be so connected.